The naturally occurring mineral, sodium sesquicarbonate, EQU Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O
is a well known source of sodium compounds, particularly useful for the production of soda ash for the glass industry. This mineral, commonly referred to as trona, occurs in large deposits in Wyoming. These deposits are approximately 85% to 95% sodium sesquicarbonate, with the remainder consisting of insoluble impurities such as oil shales, shortite (Na.sub.2 CO.sub.3.2CaCO.sub.3), and miscellaneous sedimentary materials. The water soluble constituents other than sodium sesquicarbonate are mostly other types of sodium compounds, such as sodium chloride and sodium sulfate.
The sodium content can be readily extracted from the ore by dissolution, separation of the insoluble materials from the solution, and evaporation and cooling of the solution to crystallize therefrom the sodium carbonate values in different forms. The crystals are then calcined to produce soda ash. This process produces a highly refined product by a complicated and expensive technique. Many industries, such as the glass industry, do not always require such a high grade of soda ash. Certain levels of impurities are acceptable, depending on the quality of glass to be produced. In fact, the major raw materials used in the manufacture of clear glass other than soda ash all have substantially the same impurities associated with crude trona. Beneficiation, or purification, of the trona is necessary, however, before it can be used for glass manufacture, since the amount of impurities varies and products of consistent quality cannot otherwise be obtained.
Various methods have been used to purify crude trona. In one such method, mined trona is crushed to about 8-mesh and dissolved in a hot recirculating trona mother liquor carrying more normal carbonate than bicarbonate so that the sodium carbonate and bicarbonate in the mined trona are both dissolved. The insoluble material is settled out of the solution in clarifiers and the solution polished by filtration. Sodium sesquicarbonate is crystallized and separated from the hot solution, then calcined to soda ash. The mother liquor remaining after the crystallization can be recycled to the dissolving tanks to dissolve more crude trona. A portion of the mother liquor can be passed to the crystallizers to form a second crop of sodium sesquicarbonate crystals. This processing route contains several drawbacks. First, the carbonate-bicarbonate ratio in the recycling mother liquor must be maintained. Second, the relatively low concentration of salts per unit of solution entails the use of costly large scale processing equipment to recover the sesquicarbonate. Sequestrants are required to facilitate filtration, and crystallizing aids are needed to provide particles of the desired size and shape. Both of these additives contaminate the product. Processes of this type are disclosed in U.S. Pat. Nos. 2,346,140; 2,639,217; 2,798,790; and 3,028,215.
In an alternative method, the mined trona is crushed, calcined, dissolved in an aqueous solution, clarified and filtered. The clear filtered solution is then evaporated to form sodium carbonate monohydrate crystals which are separated from the mother liquor. This mother liquor is either recycled to the crystallizers or a portion thereof is returned to dissolve more calcined trona. The monohydrate crystals are then calcined to dense ash. This route for producing soda ash has a heavy evaporation load, entailing a high capital cost for evaporative equipment. Processes of this type are disclosed in U.S. Pat. Nos. 2,343,080; 2,343,081; 2,962,348; 3,131,996; and 3,260,567.
A further alternative involves the preparation of anhydrous sodium carbonate by maintaining the temperature in the crystallization units above about 109.degree. C., which is the transition temperature at which anhydrous sodium carbonate is formed as the stable crystal phase. See U.S. Pat. No. 2,770,524. Still another method involves the preparation of sodium bicarbonate which in turn may be calcined and converted to dense sodium carbonate. A procedure of this type is disclosed in U.S. Pat. No. 2,704,239.
Each of these processing techniques involves dissolution, clarification, filtration and crystallization, with expensive reagents and recovery steps, adding substantially to the cost of the final product.
Accordingly, it is an object of the present invention to provide a relatively simple and inexpensive process for the production of soda ash from trona ore.
Further objects will be apparent from the following description.